Measurement instruments, such as spectrum analyzers and network analyzers, employ video bandwidth filtering, averaging and other types of signal conditioning to improve measurements of applied signals. For example, video bandwidth filtering within a spectrum analyzer decreases measurement variance when signals are measured in the presence of noise. Video bandwidth filtering of a signal is most advantageous when the signal is represented using an analysis scale that is most appropriate to the nature of the applied signal. When the applied signal is a continuous-wave signal, the effect of noise is minimized when video bandwidth filtering is performed with the signal represented on a first analysis scalexe2x80x94the logarithmic scale. When the applied signal is a pulsed radio frequency (RF) signal, the measurement of the pulse envelope shape of the signal is most accurately obtained when video bandwidth filtering is performed with the signal represented on a second analysis scalexe2x80x94the linear, or voltage, scale. When the applied signal is from a communication network having spectrally separated channels, adjacent channel power measurements are most accurately obtained when video bandwidth filtering is performed with the signal represented on a third analysis scalexe2x80x94the power scale. The effect of noise on spectrum analyzer measurements is discussed in Hewlett-Packard Company""s Application Note 1303, Spectrum Analyzer Measurements and Noise.
While the effect of noise on measurements within a measurement instrument can be reduced by selecting the analysis scale most appropriate to the nature of the applied signal, the scale on which the signal is preferably displayed, the display scale, does not always correspond to the selected analysis scale. For example, since the pulse envelope shape of a pulsed RF signal is most accurately measured when video bandwidth filtering is performed on the voltage scale, the preferred analysis scale for this signal is the voltage scale. However, to achieve enough dynamic range to observe the on/off ratio of the pulse envelope on a display screen, monitor, or other output device, a logarithmic display scale is preferable. In this example, the voltage scale is the preferred analysis scale, whereas the preferred display scale is the logarithmic scale. In other circumstances, the preferred analysis scale is also different from the preferred display scale, depending on the nature of the applied signal and the characteristics of the signal that are sought to be displayed.
Presently available measurement instruments having analysis scales that are coupled to the display scales are unable to filter a representation of an applied signal on a scale that is independent of the scale on which the signal is displayed. Accordingly, there is a need for an instrument or system having analysis scale and display scale that are independently selectable.
A signal analyzer constructed according to the preferred embodiment of the present invention has an analysis scale and a display scale that are independently selectable. The signal analyzer, included within a spectrum analyzer, network analyzer or other measurement instrument or system, receives a first representation of an applied signal. From the first representation of the applied signal, the signal analyzer provides at least two of a second representation, third representation and a fourth representation, the third representation being a square of the second representation, and the fourth representation being logarithmically related to the second representation and the third representation. The second, third and fourth representations correspond to alternative analysis scales, such as the voltage scale, the power scale and the logarithmic scale, respectively. A first selective input enables a designated one of the at least two representations to be applied to a filter to reduce variance of that designated representation. The signal analyzer then converts the filtered representation to at least two of a fifth, sixth and seventh representation, the sixth representation being a square of the fifth representation, and the seventh representation being logarithmically related to the fifth representation and the sixth representation. The fifth, sixth and seventh representations correspond to alternative display scales, such as the voltage scale, power scale and logarithmic scale, respectively. A second selective input enables a designated one of the fifth, sixth and seventh representations to be displayed on a monitor, display screen or other output device of the measurement instrument or system.
Independent control of the first selective input and the second selective input enables the analysis scalexe2x80x94the scale to which the filter is applied, to be selected independently of the scale on which the representation having reduced variance is displayed. A user interface provides for independent control of the first selective input and the second selective input using scale selection menus. The scale selection menus provide for optional coupling between the first selective input and the second selective input according to predefined coupling rules.